1. Field of the Invention
This invention relates generally to a focus control system and, more particularly, to a focus control system used in a contrast detecting system of a video camera.
2. Description of the Background
In an autofocusing system of the kind used in a video camera, there has been known a system for obtaininq a correct focus position that involves what is called peak detecting control. One such autofocusing system is based on the fact that selected frequency components in the video signal, excluding the DC component, become a maximum at the correct focus position. The frequency components, excluding the DC component, in the video signal are integrated and become the so-called evaluation value data and the lens is then moved to the position at which the evaluation value data becomes a maximum. An example of such a system is described in Japanese Patent application No. 146628/1987. In that kind of autofocusing system, in order to move the lens to the focus position at which the evaluation value data becomes a maximum, several processes must be executed in which the evaluation value data at the front and back lens positions are compared and a discrimination made, in order to determine the point at which the evaluation value data changes from increasing to decreasing; such processes are called peak detecting control.
FIG. 1 represents an example of such an autofocusing system, in which a lens 51 is moved by a lens drive motor 52, and an image obtained through lens 51 is picked up by a CCD image pickup device 53. The output of CCD image pickup device 53 is supplied to a signal processing circuit 54 and a luminance signal Y is extracted. The luminance signal Y is fed through a bandpass filter 55 to a detecting circuit 56, whose output is supplied to an analog-to-digital (A/D) converter 57. The digital output of A/D converter 57 is supplied to an integrating circuit 58, where the output of the A/D converter 57 is integrated over a predetermined area. The output of the integrating circuit 58 then becomes the evaluation value data. The evaluation value data is supplied to a controller 60 that produces a drive signal supplied to lens drive motor 52 through a driver interface unit 61.
Controller 60 controls the position of lens 51 by moving it to the position at which the evaluation value data, which is output from integrating circuit 58, becomes a maximum. As described above, this is peak detecting control being used as an autofocusing system. Such peak detecting control assumes that the relation between the lens position and the evaluation value data can be described by a curve, such as FIG. 2. In FIG. 2, evaluation value data Dn that is obtained at a lens position ln and evaluation value data Dn+1 that is obtained at a lens position ln+1, which is a subsequent continuous lens position relative to the lens position ln, are compared while moving lens 51 in one direction. Lens 51 is moved until the evaluation value data Dn obtained at the lens position ln is smaller than the evaluation value data Dn+1 obtained at the subsequent continuous lens position ln+1.
As shown in FIG. 2, when the lens position moves to the right, the evaluation value data increases until the lens passes through a lens position indicated as 1 focus, at which point the evaluation value data reaches a maximum value Dmax. When the lens passes through the lens position 1 focus, at which the evaluation value data has a maximum value Dmax, the evaluation value data changes in the decreasing direction. Therefore, by performing the peak detecting control such that a discrimination is made to determine whether the evaluation value data Dn and Dn+1 at the two lens positions ln and ln+1 change from increasing to decreasing while moving the lens in one direction, it is possible to determine that the lens has passed through the lens position 1 focus at which the evaluation value data teaches its maximum value Dmax, so that the correct focus position can be derived.
Nevertheless, though this method works in theory there is a situation that occurs in practice wherein a concave/convex portion appears in the evaluation value data curve of FIG. 2 that indicates a variation in the relation between a change in lens position versus a change in evaluation value data. This concave/convex portion of the response curve is due to noise and to the influence of vibrations caused by the person holding the camera, as well as other outside influences on the mechanical structure of the camera and on the electronic components. In the case where such a so-called concave/convex portion occurs in the curve, if the peak detecting control is operated to discriminate whether the evaluation value data at the two continuous lens positions have changed from increasing in value to decreasing in value while moving the lens in one direction, as explained above, there is the distinct possibility that the so-called concave/convex portion will be erroneously thought to be the lens position at which the evaluation value data is a maximum.
For example, as shown in FIG. 3, when evaluation value data Da derived at a lens position 1a and evaluation value data Da+1 derived at a lens position la+1 are compared, the evaluation value data Da+1 at the lens position la+1 is found to be smaller than the evaluation value data Da at the lens position la. Therefore, upon determining that the evaluation value data at the two successive lens positions changes from increasing to decreasing, it is determined that at this position the lens has passed through the point at which the evaluation value data becomes maximum. An erroneous focus position has then been found.
Therefore, a predetermined threshold value is provided when discriminating whether the evaluation value data at the two successive lens positions changes from increasing to decreasing. In this way, a check is made to determine whether the difference between the evaluation value data at the two lens positions has been reduced by the predetermined threshold value. If such difference exceeds the predetermined threshold value and changes in the decreasing direction, it is determined that the lens has actually passed through the lens position at which the evaluation value data is a maximum.
When using peak detecting control to perform the autofocus operation, the position of the lens at which the evaluation value data is detected as a maximum can overrun or overshoot the actual lens position at which the evaluation value data is the maximum. Therefore, a process to return the position of the lens by only the overrun amount must be performed.
If the amount of overrun that occurs when detecting the lens position at which the evaluation value data becomes a maximum is large, an undesirable oscillation occurs in the video signal on the screen. Therefore, it is desirable to reduce the amount of lens overrun past the actual focus point.
As might be assumed, this overrun amount increases when rotating the lens drive motor 52 of FIG. 1 at a high speed. Therefore, with a view to reducing the overrun amount, it is desirable to reduce the speed of rotation of the drive motor when executing peak detecting control, however, when the lens drive motor is operated at a low speed, the control speed and autofocus system response is slow.
On the other hand, as mentioned above, if a predetermined threshold value is provided when discriminating whether the evaluation value data at the two successive lens positions have changed from increasing to decreasing, it is detected that the evaluation value data has passed through the maximum value only when the evaluation value data has actually exceeded the maximum value and has decreased to the threshold value or less; consequently, the overrun amount increases.